Process for producing polyethylene pipe resins

ABSTRACT

A polyethylene resin comprising from 35 to 49 wt. % of a first polyethylene fraction of high molecular weight and from 51 to 65 wt. % of a second polyethylene fraction of low molecular weight, the first polyethylene having a density of up to 0.930 g/cm 3 , and an HLMI of less than 0.6 g/10 min and the second polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm 3  and an MI 2  of greater than 10 g/10 min, and the polyethylene resin, having a density of greater than 0.946 g/cm 3 , an HLMI of from 1 to 100 g/10 min, a dynamical viscosity, measured at 0.01 radians/second, greater than 200,000 Pa·s and a ratio of the dynamical viscosities measured at, respectively 0.01 and 1 radians/second greater than 8.

This application is a divisional application of U.S. application Ser. No. 10/415,209, filed Mar. 29, 2004, as the National Stage of International Application No. PCT/EP01/12417, filed Oct. 25, 2001 now U.S. Pat. No. 6,946,521.

The present invention relates to polyethylene resins, especially to those suitable for use as pipe resins, and to a process for producing such resins. The present invention also relates to the use of polyethylene resins for the manufacture of pipes. The present invention yet further relates to polyethylene pipes.

Polyolefins such as polyethylenes which have high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefins having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of high molecular weight fraction with the improved processing properties of the low molecular weight fraction.

For many HDPE applications, polyethylene with enhanced toughness, strength and environmental stress cracking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processibility of the resin decreases. By providing a polymer with a broad or bimodal MWD, the desired properties that are characteristic of high molecular weight resin are retained while processability, particularly extrudability, is improved.

There are several methods for the production of bimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts. Melt blending suffers from the disadvantages brought on by the requirement of complete homogenisation and high cost. Use of a dual site catalyst for the production of a bimodal resin in a single reactor is also known.

Metallocene catalysts are known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylenes having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system which includes at least two metallocenes is employed. The metallocenes used are, for example, a bis(cyclopentadienyl) zirconium dichloride and an ethylene-bis(indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained which is at least bimodal.

EP-A-0881237 discloses the production of bimodal polyolefins with metallocene catalysts in two reaction zones. The metallocene catalyst component comprises a bis-tetrahydro indenyl compound of the general formula (IndH₄)₂R″MQ₂ in which each IndH₄ is the same or different and is tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridge which comprises a C₁–C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen. That specification discloses that the density of the multimodal polyolefin resins particularly falls in the range 0.9 to 0.97 g/ml, preferably 0.92 to 0.97 g/ml and that the HLMI of the polyolefin resins particularly falls within the range 0.1 to 45,000 g/10 min, preferably in the range 0.4 to 45,000 g/10 min. Thus, that specification discloses the production of polyolefin resins having a wide variety of properties.

EP-A-0989141 also discloses a process for the preparation of polyethylenes having a multimodal molecular weight distribution. The catalyst may employ a metallocene catalyst comprising a bis-tetrahydroindenyl compound as disclosed in EP-A-0881237. The specification discloses the production of pipe resins. Although the pipe resins disclosed had good mechanical properties, there is still a need to improve the mechanical properties. There is a need to produce a polyethylene resin having improved mechanical properties yet with good processibility.

Polyethylene resins are known for the production of pipes. Pipe resins require high resistance against slow crack growth as well as resistance to crack propagation yielding impact toughness.

Pipe resins are known in the art which are referred to by the names “PE 80” and “PE 100”. These are polyethylene resins which when formed into pipes of specific dimensions, survive a long term pressure test at different temperatures for a period of 5,000 hours. Extrapolation shows that they have a 20° C.—50 years resistance of at least 8 and 10 MPa, respectively. This classification is described in ISO 9080 and ISO 12162. It is known in the art that the key components for a good PE 100 resin are the blending of a low molecular weight high density polyethylene with little or no short chain branching (SCB) due to comonomer incorporation and a linear low density polyethylene (LLDPE) resin with high molecular weight and SCB. Known pipe resins have a trade off between mechanical properties and processibility. Despite this, there is still a need to improve known pipe resins.

Usually, polyethylene pipe resins in the form of chemical blends are produced in a cascade reactor process using Ziegler-Natta catalysts.

These known PE100 resins have in general a rheological behaviour that could be improved. They generally have a relatively low viscosity at low shear rates. In particular, the difference between their viscosity at low shear rate and their viscosity at high shear rate is rather small. This means that during the extrusion of these resins for the manufacture of pipes, sagging can occur. Moreover the injection-moulding capability for the known PE100 resins is not optimal and renders them more difficult to use for the production of injection moulded pipe fittings.

The present invention aims to overcome the disadvantages of the prior art, in particular by providing improved polyethylene pipe resins.

The present invention provides a polyethylene resin comprising from 35 to 49 wt % of a first polyethylene fraction of high molecular weight and from 51 to 65 wt % of a second polyethylene fraction of low molecular weight, the first polyethylene fraction comprising a linear low density polyethylene having a density of up to 0.930 g/cm³, and an HLMI of less than 0.6 g/10 min and the second polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm³ and an MI₂ of greater than 10 g/10 min, and the polyethylene resin having a density of greater than 0.946 g/cm³, an HLMI of from 1 to 100 g/10 min, a dynamical viscosity η_(0.01), measured at 0.01 radian/second, greater than 200,000 Pa·s and a ratio of the dynamical viscosities measured at, respectively 0.01 and 1 radian/second, η_(0.01)/η₁ greater than 8.

The present invention further provides the use of such a polyethylene resin for the manufacture of pipes and fittings.

The invention also provides a pipe or a fitting comprising the polyethylene resin of the invention.

The present invention further provides a process for the preparation of a polyethylene resin having a bimodal molecular weight distribution which comprises:

(i) contacting ethylene monomer and a first co-reactant with a catalyst system in a first reaction zone under first polymerisation conditions to produce a first polyethylene; and

(ii) contacting ethylene monomer and a second co-reactant with a catalyst system in a second reaction zone under second polymerisation conditions to produce a second polyethylene different from the first polyethylene;

wherein the first and second polyethylenes are blended together, to form a polyethylene resin comprising a blend of from 35 to 49 wt % of a first polyethylene fraction of high molecular weight and from 51 to 65 wt % of a second polyethylene fraction of low molecular weight, the first polyethylene fraction comprising a linear low density polyethylene having a density of up to 0.930 g/cm³, and an HLMI of less than 0.6 g/10 min and the second polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm³ and an MI₂ of greater than 10 g/10 min, and the polyethylene resin having a density of greater than 0.946 g/cm³, an HLMI of from 1 to 100 g/10 min, a dynamical viscosity η_(0.01), measured at 0.01 radians/second, greater than 200,000 Pa·s and a ratio of the dynamical viscosities measured at, respectively 0.01 and 1 radians/second, η_(0.01)/η₁ greater than 8, wherein one of the co-reactants is hydrogen and the other is a comonomer comprising a 1-olefin containing from 3 to 12 carbon atoms each catalyst system comprising (a) a metallocene catalyst component comprising a bis tetrahydroindenyl compound of the general formula (IndH₄)₂R″MQ₂ in which each IndH₄ is the same or different and is tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridge which comprises a C₁–C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a cocatalyst which activates the catalyst component.

The polyethylene resins in accordance with the invention have a dynamical viscosity η_(0.01), measured at 0.01 radian/second, which is greater than 200,000 Pa·s. In contrast, known pipe resins produced using Ziegler-Natta catalysts have a η_(0.01) less than 200,000 Pa·s.

In addition, the polyethylene resins in accordance with the invention have a η_(0.01)/η₁ ratio greater than 8, preferably greater than 10, where η₁ is the dynamic viscosity at 1 radian/second, expressed in Pa·s. In contrast, known pipe resins produced using a Ziegler-Natta catalyst have a η_(0.01)/η₁ ratio typically much less than 8, most typically around 5.

The determination of dynamical viscosity is made by using an oscillatory rheometer, preferably a Rheometric Scientific ARES rheometer. This method has been extensively described in the literature devoted to polymer rheology (see e.g. W. W. Graessley, Chapter 3 in Physical Properties of Polymers, 2nd Edition, ACS Professional Reference Book, Washington D.C., 1993).

The measurements are performed on a Rheometric Scientific ARES rheometer between two 25 mm diameter plates; the gap between the plates is between 1 and 2 mm, and is thoroughly adapted according to the suitable thickness of the polymer sample once this latter has been inserted between the plates and warmed up to 190° C. The gap value is then recorded to be taken into account by the calculation software.

The sample is then temperature-conditioned for a period of 5 minutes before the measurement is started. The measurement is performed at 190° C. After temperature conditioning, the measurement starts by applying an oscillatory strain γ*(ω,t)=γ_(M).e^(iωt), with a given amplitude γ_(M) and a given frequency ω to the bottom plate via a precision motor, whereas the top plate is kept fixed. The amplitude γ_(M) of this shear strain has been chosen in the linear zone of viscoelasticity of the polymer and is kept constant through the whole experiment. The oscillation frequency ω is varied through the range [10⁻²–10⁺²] radians/second. The oscillating shear strain is translated inside the material into an oscillating shear stress σ*(ω,t), which in-phase and out-of-phase components are recorded as functions of the frequency ω, and used for the calculation of the complex modulus G*(ω) as well as complex viscosity η*(ω) of the polymer:

$\begin{matrix} {{G^{*}(\omega)} = {\frac{\sigma^{*}\left( {\omega,t} \right)}{\gamma^{*}\left( {\omega,t} \right)} = {{{G_{m}(\omega)} \cdot {\mathbb{e}}^{{\mathbb{i}}\;{\delta{(\omega)}}}} = {{G^{\prime}(\omega)} + {i \cdot {G^{''}(\omega)}}}}}} \\ \begin{matrix} {{{G_{m}(\omega)} = \sqrt{{G^{\prime 2}(\omega)} + {G^{''2}(\omega)}}};} & {{\tan\mspace{11mu}\delta\;(\omega)} = \frac{G^{''}(\omega)}{G^{\prime}(\omega)}} \end{matrix} \\ {{\eta^{*}(\omega)} = {{{\eta^{\prime}(\omega)} - {i \cdot {\eta^{''}(\omega)}}} = {\frac{G^{''}(\omega)}{\omega} - {i \cdot \frac{G^{\prime}(\omega)}{\omega}}}}} \\ {{{\eta^{*}(\omega)}} = \frac{\sqrt{{G^{\prime 2}(\omega)} + {G^{''\; 2}(\omega)}}}{\omega}} \end{matrix}$

According to the Cox-Merz rule, the function of the absolute value of the complex viscosity ∥η*(ω)∥ is the same as the conventional viscosity function, (capillary viscosity as a function of shear rate γ), if frequency is taken in rad/s. If this empiric equation is valid, the absolute value of the complex modulus corresponds to the shear stress in conventional (that is steady state) viscosity measurements.

In the present invention, the dynamic viscosities of the resin measured at 0.01 and 1 rad/s respectively according to the aforementioned method are defined as η_(0.01)=∥η*(0.01 rad/s)∥ and η₁=∥η*(1 rad/s)∥.

The polyethylene resins in accordance with the invention preferably satisfy the following relationship: η_(0.01)/η₁≧{(0.293×M _(w) /M _(n))+3.594}

The polyethylene resins in accordance with the invention preferably satisfy the following relationship: η_(0.01)/η₁≧{(−0.302×HLMI)+9.499}

The resin according to the present invention preferably comprises at least 55% by weight of the second polyethylene fraction of low molecular weight, most preferably at least 56 weight %.

The resin according to the present invention preferably comprises not more than 45% by weight of the first polyethylene fraction of high molecular weight, most preferably at most 44 weight %.

In this specification the melt index MI₂ and high load melt index HLMI are measured in accordance with ASTM D-1238 at 190° C. with respective loads of 2.16 and 21.6 kg.

Preferably, for the high density polyethylene fraction the MI₂ is from 100 to 1000 g/10 min, more preferably from 300 to 1000 g/10 min.

Preferably, for the low density polyethylene fraction, the HLMI is from 0.001 to 0.5 g/10 min, more preferably from 0.01 to 0.25 g/10 min.

Preferably, for the polyethylene resin, the HLMI is from 5 to 90 g/10 min, more preferably from 10 to 80 g/10 min.

In this specification the density is measured in accordance with ISO 1183.

For the low density polyethylene fraction, the density is preferably from 0.908 to 0.927 g/cm³, more preferably from 0.912 to 0.926 g/cm³.

For the high density polyethylene fraction, the density is preferably from 0.970 to 0.990 g/cm³, more preferably from 0.971 to 0.980 g/cm³.

Preferably, the density of the polyethylene resin is from 0.949 to 0.960 g/cm³, more preferably from 0.952 to 0.958 g/cm³.

Particularly preferred is a resin having a density of at least 0.954 g/cm³.

For the high density polyethylene fraction, the polydispersity index D (represented by the ratio Mw/Mn as determined by gel permeation chromatography (GPC)) is preferably from 2 to 4. For the linear low density polyethylene fraction of high molecular weight the value of polydispersity index D is preferably from 3 to 6.

Preferably, the polyethylene resin has a molecular weight distribution Mw/Mn from 8 to 30.

Preferably, the low density fraction is a copolymer of ethylene and another alpha-olefin containing from 3 to 12 carbon atoms. More preferably, the low density fraction is a copolymer of ethylene and butene, methylpentene, hexene and/or octene.

Preferably, the high density fraction is an ethylene homopolymer.

The polyethylene resins in accordance with the invention have in general a lower capillary viscosity μ₂ than commercial PE100 resins. Preferably, μ₂ is less than 21,000 dPa·s. In contrast, known pipe resins produced using Ziegler-Natta catalysts, have a high capillary viscosity which renders them rather difficult to extrude, they typically have a μ₂ greater than 21,000 dPa·s. μ₂ is the value of capillary viscosity which is measured by extruding polymer by means of an extrusion device, which incorporates a piston in a cylinder, at a temperature of 190° C. through a cylindrical die of length 30 mm and diameter 2 mm at a constant speed corresponding to a shear rate of 100 s⁻¹ and by measuring the force transmitted by the piston during the decent of the piston. The cylinder and piston used by this test method meet the requirements of the cylinder/piston device used for fluidity index measurements according to the standard ASTM D 1238 (1996). The μ₂ value is then calculated by using the equation: μ₂=23.61×Fp where Fp represents the mean force exerted by the piston during the measurement period and is expressed in decaNewtons (daN) whereas μ₂ is express in dPa·s.

The polyethylene resins according to the invention, having such a specific composition, molecular weight and density, can lead to a marked improvement of the processing properties when the resin is used as a pipe resin, while conserving or improving mechanical behaviour as compared to known pipe resins.

The polyethylene resins in accordance with the invention can outperform the current best available bimodal polyethylene resins of PE 100 grade for properties relating to the fabrication and use of polyethylene pipes.

In particular, the polyethylene resins in accordance with the invention have impact resistance and slow crack resistance at least equivalent, often higher than current available PE 100 grade resins.

The resins of the invention are endowed with excellent rheological behaviour, that is they have a similar or lower viscosity at higher shear rates (typically around 100 s⁻¹) and a much higher viscosity at low shear rates (0.1 s⁻¹ or below). These resins provide a reduced sagging following extrusion of the resin into a pipe together with an improvement of the injection moldability.

The resin in accordance with the invention is characterised by a higher shear-thinning behaviour which is better than known bimodal PE 100 resins. This means less sagging of the polyethylene resins when being extruded at low shear rates when forming pipes, and good injection-moulding capability for the resins when used to produce injection moulded pipe fittings.

The resins according to the invention permit to obtain pipes having a good resistance to slow crack growth.

In this specification in order to assess the slow crack growth resistance of pipe resins, an environmental stress cracking resistance (ESCR) Bell test in accordance with ASTM-D-1693 at 70° C., with 35% Antarox CO630 was employed, and the times to failure were measured. The resins produced in accordance with the invention have a very high ESCR resistance as measured by the Bell test, the failure times typically being greater than 400 hours, most often even greater than 500 hours.

The slow crack growth resistance of the resins was also tested by a full notch creep test (FNCT) according to ISO DIS 16770 in which the time for failure was recorded for a circumferentially notched specimen having a 10 mm×10 mm cross section, the specimen having been submitted to a net tensile strength of 5 MPa at a temperature of 80° C. in a 2% solution of Arkopal N100. Generally, the resins in accordance with the invention exhibit a time to failure under the FNCT test specified here above of at least 500 hours, indicating good slow crack growth resistance.

For some of the resins, the slow crack growth resistance was further tested by a notched pipe test (NPT) in accordance with ISO 13479 under a stress of 4.6 MPa at 80° C. using pipes of 110 mm diameter (SDR 11).

Furthermore, the resins in accordance with the invention exhibit good resistance to rapid crack propagation. In order to assess resistance of the resins to rapid crack propagation (RCP), pipes having a diameter of 110 mm (SDR11) were subjected to the test according to ISO DIS 13477 (the small scale steady state (S4) test) at a pressure of 10 bars to determine the critical temperature of fracture. A Charpy impact energy test was also carried out at a temperature of −25° C. using the procedures of ISO 180/1A.

Furthermore, the resins according to the invention have a good creep resistance. The creep resistance was measured according to the test of ISO 1167 on 32 mm diameter SDR 11 pipes to determine the lifetime prior to failure at a temperature of 20° C. and a pressure of 13 MPa.

The pipe resins according to the invention have a creep resistance measured according to ISO 1167 of at least 400 hours for a temperature of 20° C. and a pressure of 13 MPa. The resins produced in accordance with the invention generally have a creep resistance which is such that they can be assigned a minimum required strength (MRS) rating according to the ISO/TR 9080 standard which is the MRS 10 rating or even higher, such as a MRS 11.2 rating or even a MRS 12.5 rating. This rating is determined according to a statistical method and the minimum required strength MRS is defined as the lower prediction limit (LPL) at a 97.5% confidence interval.

The resins according to the invention are therefore well suited for the manufacture of pipes, in particular high pressure pipes. The present invention therefore also concerns the use of the polyethylene resins according to the invention for the manufacture of pipes and fittings and the pipes and fittings thereby obtained. When used for the manufacture of pipes, the resins are most often blended with usual additives such as anti-oxydants, anti-acids and colorants.

The polyethylene resins according to the invention can be prepared by any method suitable therefore. They can be prepared by physically blending the high density and the low density polyethylene fractions, prepared separately, or they can be prepared by polymerising ethylene in the presence of a mixture of catalysts. Preferably, the high density and low density fractions are produced in at least two separate reactors, most preferably two such reactors in series. In such a case, the high density fraction is preferably prepared first, so that the low density fraction is prepared in the presence of the high density fraction.

The catalyst employed in the polymerisation process may be any catalyst(s) suitable for preparing the low and high density fractions. Preferably, the same catalyst produces both the high and low molecular weight fractions in general, a metallocene catalyst is used. The metallocene catalyst component preferably comprises a bis-tetrahydroindenyl compound (THI) of formula (IndH₄)₂R″MQ₂ in which each IndH₄ is the same or different and is tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridge which comprises a C₁–C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen.

The preferred bis tetrahydroindenyl catalyst may be substituted in the same way or differently from one another at one or more positions in the cyclopentadienyl ring, the cyclohexenyl ring and the ethylene bridge. Each substituent group may be independently chosen from those of formula XR_(v) in which X is chosen from group IVB, oxygen and nitrogen and each R is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms and v+1 is the valence of X. X is preferably C. If the cyclopentadienyl ring is substituted, its substituent groups must not be so bulky as to affect coordination of the olefin monomer to the metal M. Substituents on the cyclopentadienyl ring preferably have R as hydrogen or CH3. More preferably, at least one and most preferably both cyclopentadienyl rings are unsubstituted.

In a particularly preferred embodiment, both tetrahydroindenyls are unsubstituted.

R″ is preferably an ethylene bridge which is substituted or unsubstituted.

The metal M is preferably zirconium, hafnium or titanium, most preferably zirconium. Each Q is the same or different and may be a hydrocarbyl or hydrocarboxy radical having 1–20 carbon atoms or a halogen. Suitable hydrocarbyls include aryl, alkyl, alkenyl, alkylaryl or aryl alkyl. Each Q is preferably halogen. Ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride is a particularly preferred bis tetrahydroindenyl compound of the present invention.

The metallocene catalyst component used in the present invention can be prepared by any known method. A preferred preparation method is described in J. Organomet. Chem. 288, 63–67 (1985).

The cocatalyst which activates the metallocene catalyst component can be any cocatalyst known for this purpose such as an aluminium-containing cocatalyst or a boron-containing codatalyst. The aluminium-containing cocatalyst may comprise an alumoxane, an alkyl aluminium and/or a Lewis acid.

The alumoxanes used in the process of the present invention are well known and preferably comprise oligomeric linear and/or cyclic alkyl alumoxanes represented by the formula:

for oligomeric, linear alumoxanes and

for oligomeric, cyclic alumoxane, wherein n is 1–40, preferably 10–20, m is 3–40, preferably 3–20 and R is a C₁–C₈ alkyl group and preferably methyl.

Generally, in the preparation of alumoxanes from, for example, aluminium trimethyl and water, a mixture of linear and cyclic compounds is obtained.

Suitable boron-containing cocatalysts may comprise a triphenylcarbenium boronate such as tetrakis-pentafluorophenyl-borato-triphenylcarbenium as described in EP-A-0427696, or those of the general formula [L′-H]+[B Ar₁ Ar₂ X₃ X₄]— as described in EP-A-0277004 (page 6, line 30 to page 7, line 7).

Preferably, the same catalyst system is used in both steps of the cascade polymerisation process to produce a chemical blend of the high and low molecular weight fractions. The catalyst system may be employed in a solution polymerisation process, which is homogeneous, or a slurry process, which is heterogeneous. In a solution process, typical solvents include hydrocarbons with 4 to 7 carbon atoms such as heptane, toluene or cyclohexane. In a slurry process it is preferably to immobilise the catalyst system on an inert support, particularly a porous solid support such as talc, inorganic oxides and resinous support materials such as polyolefin. Preferably, the support material is an inorganic oxide in its finally divided form.

Suitable inorganic oxide materials which are desirably employed in accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides such as silica, alumina and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, or alumina are magnesia, titania, zirconia, and the like. Other suitable support materials, however, can be employed, for example, finely divided functionalised polyolefins such as finely divided polyethylene.

Preferably, the support is a silica having a surface area comprised between 100 and 1000 m2/g and a pore volume comprised between 0.5 and 4 ml/g.

The amount of alumoxane and metallocenes usefully employed in the preparation of the solid support catalyst can vary over a wide range. Preferably the aluminium to transition metal mole ratio is in the range between 1:1 and 100:1, preferably in the range 5:1 and 70:1.

The order of addition of the metallocene and alumoxane to the support material can vary. In accordance with a preferred embodiment of the present invention alumoxane dissolved in a suitable inert hydrocarbon solvent is added to the support material slurried in the same or other suitable hydrocarbon liquid and thereafter a mixture of the metallocene catalyst component is added to the slurry.

Preferred solvents include mineral oils and the various hydrocarbons which are liquid at reaction temperature and which do not react with the individual ingredients. Illustrative examples of the useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane and nonane; cycloalkanes such as cyclopentane and cyclohexane; and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene.

Preferably the support material is slurried in toluene and the metallocene and alumoxane are dissolved in toluene prior to addition to the support material.

In one arrangement according to the present invention, each polyolefin is produced individually in a reactor, preferably a loop reactor, and mixed together by extrusion. The polyolefins may be mixed together by melt blending. In this way, the low molecular weight and high molecular weight parts of the polyolefin can be produced in separate reactors.

In a preferred arrangement, the product of a first cascade reaction zone, including the olefin monomer, is contacted with the second co-reactant and the catalyst system in a second cascade reaction zone to produce and mix the second polyolefin with the first polyolefin in the second reaction zone. The first and second reaction zones are conveniently interconnected reactors such as interconnected loop reactors or interconnected loop and continuously stirred reactors. It is also possible to introduce into the second reaction zone fresh olefin monomer as well as the product of the first reaction zone.

Because the second polyolefin is produced in the presence of the first polyolefin a multimodal or at least bimodal molecular weight distribution is obtained.

In one embodiment of the invention, the first co-reactant is hydrogen and the second co-reactant is the comonomer. Typical comonomers include hexene, butene, octene or methlylpentene, preferably hexene.

In an alternative embodiment, the first co-reactant is the comonomer, preferably hexene. Because the metallocene catalyst components of the present invention exhibit good comonomer response as well as good hydrogen response, substantially all of the comonomer is consumed in the first reaction zone in this embodiment. Homopolymerisation takes place in the second reaction zone with little or no interference from the comonomer.

In another embodiment, hydrogen may be introduced in both the first and the second reactor.

The temperature of each reactor may be in the range of from 60° C. to 110° C., preferably from 70° C. to 90° C.

The invention will now be described in further detail with reference to the following non-limiting Examples.

EXAMPLE 1

A. Preparation of the Catalyst

The catalyst comprises ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium chloride metallocene catalyst supported on a silica support which had been treated with a cocatalyst comprising methylalumoxane (MAO).

Silica having a total pore volume of 1.56 ml/g and a surface area of 311 m²/g was dried in a fluidised bed reactor for a period of 6 hours at a temperature of 150° C. under a nitrogen flow of 75 Nl/h. An amount of 10 g of the dried silica suspended in 100 ml of dry toluene at room temperature was introduced via a dropping funnel into a 500 ml round bottom flask equipped with a magnetic stirrer, and maintained under nitrogen gas. Thereafter, 40 ml of a 30 wt % solution of methylalumoxane (MAO) in toluene (thereby providing a 1.1 weight ratio between MAO and the silica) were slowly added to the silica suspension. The solution of MAO in toluene is commercialy available from the company Witco. The resulting slurry was heated at a temperature of 110° C. for a period of 4 hours. The solid reaction product was then isolated by filtration, using a frit funnel of porosity value 3, and then washed three times with 100 ml of dried toluene and then three times with 100 ml of dry pentane. The catalytic support was then finally dried under vacuum to produce 21 g of a free flowing white powder.

Thereafter, for the deposition of the metallocene catalyst onto the support as described above, 19 g of the catalytic support was suspended in 200 ml of dry toluene in a 500 ml round bottom flask equipped with a magnetic stirrer and maintained under nitrogen. Then 1.21 g of ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, also commercially available from the company Witco, were added to the suspension. The amount of the metallocene compound was selected so as to provide 6 wt % of the metallocene catalyst on the support, based on the weight of the support. The resulting yellow slurry was allowed to react for a period of 2 hours. The supported catalyst was then filtered off and washed with successive portions of 100 ml of dry toluene until a colourless filtrate was obtained. The catalyst was then washed three times with 100 ml of dry pentane and dried under vacuum. The resultant supported catalyst was obtained as 20.2 g of a pale yellow free flowing powder.

B. Preparation of the Low Molecular Weight (LMW) Polyethylene Fraction

Under a flushing of dry nitrogen gas 0.9 millimole of tri-isobutyl aluminium (TIBAL) and 900 ml of isobutane were introduced into a dry autoclave reactor having a volume of 3 litres and provided with an agitator. The temperature was raised to 80° C. and hydrogen gas was added. Ethylene gas was then introduced until a partial pressure of ethylene of 10×10⁵ Pa was achieved. The amount of hydrogen previously introduced into the autoclave reactor was selected so as to obtain a final gas phase molar ratio of hydrogen to ethylene (H₂/C₂ molar ratio) of 0.0029 mol/mol.

The polymerisation was then started by flushing into the autoclave the solid catalyst, comprising 6 wt % of the supported ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride metallocene catalyst (the THI catalyst) prepared hereinabove, in 100 ml of isobutane. The temperature, partial pressure of ethylene, and the H₂/C₂ ratio were kept constant over the polymerisation period, which was 1 hour. The reaction was stopped by cooling down, and then venting, the reactor.

The low molecular weight polyethylene was then collected from the reactor. The properties of the low molecular weight polyethylene obtained are summarised in Table I.

C. Preparation of the High Molecular Weight (HMW) Polyethylene Fraction

The process for preparing the high molecular weight fraction was the same as that for preparing the low molecular weight fraction specified above in step B, except that instead of adding hydrogen after raising the temperature to 80° C., 10 g of 1-hexene comonomer were added and a different amount of ethylene was introduced, in order to obtain a partial pressure of ethylene of 8×10⁵ Pa.

The high molecular weight ethylene-hexene copolymer obtained was collected from the reactor. The properties of the high molecular weight polyethylene fraction are summarised in Table I.

D. Preparation of the Polyethylene Resin Blend

In order to prepare the blend of the low molecular weight and high molecular weight polyethylene fractions, 550 g of the low molecular weight polyethylene fraction obtained in step B were then blended with 450 g of the high molecular weight ethylene-hexene copolymer obtained in step C. The resulting blend was pelletised twice in an extruder, available in commerce from the company APV Baker under the trade name MP19TC25.

The properties of the blended polyethylene resin thereby obtained are summarised in Tables I and II.

EXAMPLE 2

In Example 2, the same metallocene catalyst as in Example 1 was employed to produce both low molecular weight and high molecular weight polyethylene resin fractions. However, the polymerisation conditions, in particular the molar ratio of hydrogen to ethylene (H₂/C₂), the 1-hexene content and the ethylene partial pressure in steps B and C were varied in order to obtain ethylene polymers having different densities, and in addition the proportion of the low molecular weight and high molecular weight polyethylene fractions used for making the polyethylene resin blend were changed. The composition and properties of the polyethylene resins obtained are summarised in Tables I and II.

Comparative Examples 1 to 15

In these Comparative Examples, Example 1 was again repeated except that the polymerisation conditions, namely the molar ratio of hydrogen to ethylene (H₂/C₂), the 1-hexene content and the ethylene partial pressure in steps B and C were adapted in order to obtain ethylene polymers having different densities and/or melt indices and/or except that the proportions of the low molecular weight and high molecular weight polyethylene fractions used for making the polyethylene resin blend were changed. The composition and properties of the polyethylene resins thereby obtained are summarised also in Tables I and II.

EXAMPLE 3

A. Preparation of Metallocene Catalyst

The metallocene catalyst was the same as that employed in Example 1.

B. Preparation of the Low Molecular Weight (LMW) Polyethylene Fraction

An amount of 0.9 millimole of TIBAL and 900 ml of isobutane were introduced under a flushing of dry nitrogen into a dry autoclave reactor having a volume of 3 liters and provided with an agitator. The temperature was raised to 80° C. and hydrogen gas was added in order to obtain a final hydrogen/ethylene (H₂/C₂) molar ratio of 0.0058 mol/mol. Ethylene was then introduced until a partial pressure of ethylene of 10×10⁵ Pa was obtained.

The polymerisation was started by flushing into the autoclave reactor the catalytic solid which was the same as that employed in Example 1 namely comprising 6 wt % of ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride on the MAO treated silica support in 100 ml of isobutane. The temperature, partial pressure of ethylene, and hydrogen/ethylene ratio was kept constant during the polymerisation period in order to obtain the desired amount of homopolymer. The reactor was then cooled down and vented until a total pressure of 0.5×10⁵ Pa was reached. A very small sample of the low molecular weight polyethylene fraction was taken from the reactor in order to measure the density and melt index (MI₂) of the fraction. The reminder of the low molecular weight polyethylene fraction was left in the reactor.

C. Preparation of the High Molecular Weight (HMW) Polyethylene Fraction in the Presence of the Low Molecular Weight Fraction obtained in Step B.

The temperature of the reactor containing the low molecular weight polyethylene fraction was then raised up again to 80° C. and 10 g of 1-hexene were added. Ethylene was then introduced again into the reactor in order to obtain the desired partial pressure of ethylene of 10×10⁵ Pa. The temperature and partial pressure of ethylene were kept constant during the polymerisation period in order to obtain the desired amount of copolymer, thereby yielding the desired weight ratio between on the one hand the homopolymer produced in the first polymerisation step and on the other hand the copolymer produced in the second copolymerisation step. The reaction was then stopped by cooling down, and then venting, the reactor.

The resulting polyethylene resin comprising the low molecular weight and high molecular weight fractions chemically blended together, was then collected from the reactor.

D. Pelletisation of the Polyethylene Resin

The polyethylene resin was pelletised using an extruder the same as that employed in Example 1. The properties of the pelletised resin blend are summarised in Tables III and IV.

EXAMPLES 4 to 6

The operations of Example 3 were repeated, except that the polymerisation conditions (the molar ratio of hydrogen to ethylene, the 1-hexene content and the ethylene partial pressure) in steps B and C were varied in order to obtain ethylene polymers having different densities and/or melt indices, and except that the proportions of the low molecular weight and high molecular weight polyethylene fractions in the blend were changed by varying the duration of the first and second polymerisation steps. The composition and properties of the resultant polyethylene resins thereby obtained are summarised also in Tables III and IV.

Comparative Examples 16 to 19

In these Comparative Examples, the operations of Example 3 were again repeated, except that the polymerisation conditions (molar ratio) of hydrogen to ethylene, 1-hexene content, and ethylene partial pressure in the first and second polymerisation steps were adapted in order to obtain ethylene polymers having different densities and/or melt indices and/or except that the proportions of the low molecular weight and high molecular weight polyethylene fractions of the blend were changed by adapting the duration of the first and second polymerisation steps.

The composition and properties of the resultant polyethylene resins are summarised in Tables III and IV.

EXAMPLE 7

A. Catalyst Preparation

Ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride was prepared in accordance with the method of Brintzinger as published in Journal of Organometallic Chemistry, 288 (1985) pages 63 to 67.

The support used in a silica having a total pore volume of 4.217 ml/g and a surface area of 322 m²/g. This silica was further prepared by drying in high vacuum on a schlenk line for three hours to remove the physically absorbed water. 5 g of this silica were suspended in 50 ml of toluene and placed in a round bottom flask equipped with magnetic stirrer, nitrogen inlet and dropping funnel.

An amount of 0.31 g of the metallocene was reacted with 25 ml of methylalumoxane (MAO 30 wt % in toluene) at a temperature of 25° C. for a period of 10 minutes to give a solution mixture of the corresponding metallocenium cation and the anionic methylalumoxane oligomer.

Then the resulting solution comprising the metallocenium cation and the anionic methylalumoxane oligomer was added to the support under a nitrogen atmosphere via the dropping funnel which was replaced immediately after with a reflux condenser. The mixture was heated to 110° C. for 90 minutes. Then the reaction mixture was cooled down to room temperature, filtered under nitrogen and washed with toluene.

The catalyst obtained was then washed with pentane and dried under a mild vacuum.

B. Preparation of the Polyethylene Resin

The manufacture of a polyethylene resin comprising a blend of low molecular weight and high molecular weight ethylene polymers was carried out in suspension in isobutane in two loop reactors connected in series.

Isobutane, ethylene, hydrogen, triisobutyl aluminium (TIBAL) and the catalyst (prepared according to the procedure described in section A above) were continuously introduced into the first loop reactor and the polymerisation of ethylene was carried out in this mixture in order to form the low molecular weight homopolymer. The polymerisation conditions are specified in Table V. The mixture, additionally comprising the low molecular weight homopolymer, was continuously withdrawn from the first loop reactor and was subjected to a reduction in pressure, so as to remove the hydrogen. The resulting mixture was then continuously introduced into a second loop reactor serially connected to the first loop reactor together with ethylene, 1-hexene and isobutane. The polymerisation of the ethylene and 1-hexene was carried out therein in order to form the high molecular weight copolymer. The suspension comprising the polyethylene resin blend of the low molecular weight and high molecular weight ethylene polymer fractions was continuously withdrawn from the second loop reactor. The suspension was subjected to a final reduction in pressure, so as to evaporate the isobutane and the remaining reactants present (ethylene, 1-hexene and hydrogen) and to recover the polyethylene resin in the form of a powder, which was subject to drying in order to complete the de-gasing of the isobutane. The polymerisation conditions in the second reactor are also specified in Table V.

The properties of both the low molecular weight and high molecular weight polyethylene resin fractions and also of the ultimate polyethylene resin, are specified in Table VI, together with further measurements of the capillary viscosity and dynamic viscosity of the ultimate polyethylene resin blend.

C. Use of the Composition for the Preparation of Pipes

The polyethylene resin obtained in step B was mixed with 3000 ppm by weight of an anti-oxidising agent, which comprised the compound having the trade name Irganox B225 available in commerce from the company CIBA Speciality Chemicals and with 2.25% by weight of carbon black. The resultant compound was granulated by extrusion in a twin screw extruder available under the trade name ZSK 58 from the company Werner & Pfleiderer at a temperature of 215° C.

Thereafter, pipes were manufactured by extrusion of these granules through a single screw extruder at a temperature of 200° C. Pipes having a diameter of 110 mm were extruded through a single screw extruder available in commerce from the company Krauss Mafei under the trade name KME 1-70-30B. These pipes had an SDR value of 11, the SDR being the ratio of external diameter to thickness. Also, 32 mm pipes were produced on a Reifenhauser extruder.

The resins were tested to determined the environmental stress crack resistance (ESCR) using the FNCT and notched pipe tests, the creep resistance, the rapid crack propagation and the Charpy impact energy. The results are summarised in Table VII.

EXAMPLE 8

Example 7 was repeated by using different polymerisation conditions in both the first and second reactors and the different conditions are summarised in Table V. The resultant properties of the low molecular weight and high molecular weight fractions and also of the ultimate resin are also summarised in Table VI. Table VI also specifies the capillary and dynamic viscosity of the ultimate resin. The resin was also subjected to the same ESCR (using the FNCT and notched pipe tests), creep resistance, rapid crack propagation and Charpy impact energy tests as the resin of Example 7 and the results are summarised in Table VII.

Comparative Example 20

The polyethylene resin of this Comparative Example comprised a commercially available PE100 compound and the properties of the compound are summarised in Tables VI and VII.

Comparison of Results

From a comparison of Examples 1 and 2 with Comparative Examples 1 to 15 it may be seen that the resin in accordance with the invention has a significantly higher environmental stress cracking resistance as measured by the Bell test than the resin blends not produced in accordance with the invention. Furthermore, from a comparison of Examples 3 to 6 with the results of Comparative Examples 16 to 19, it may also be seen that the environmental stress cracking resistance as measured both by the Bell test and by the FNCT test is significantly enhanced with the resin blends produced in accordance with the present invention. Furthermore, when the resin of Examples 7 and 8 is compared to the resin of Comparative Example 20, corresponding to a commercially available PE100 compound, it may be seen that the resin produced in accordance with the invention has good environmental stress crack resistance as determined both by the FNCT test and the notch pipe test, and significantly higher creep resistance than that of the Comparative Example. In addition, the resin produced in accordance with the invention has resistance to rapid crack propagation and a Charpy impact energy which substantially correspond to or are greater than those of the known commercially available PE100 resin.

Furthermore, it may be seen that the polyethylene resins of the invention have a capillary viscosity μ₂ lower than that of commercial PE100 resins which are Ziegler-Natta pipe resins. Accordingly, at higher shear rates the resins of the invention can exhibit improved processability as compared to the known PE100 resins. In addition, the polyethylene resins produced in accordance with the invention also have an η_(0.01) significantly greater than 200,000 Pa·s, the typical maximum value for commercially available PE100 Ziegler-Natta pipe resins. Accordingly at lower shear rates the resins of the invention can exhibit improved resistance to sagging for extruded pipes, for example, as compared to known PE100 resins. In addition, the resins produced in accordance with the invention, in general have an η_(0.01)/η₁ greater than 8, most often greater than 10, which is greater than the maximum value of 8 typically exhibited by commercially available PE100 Ziegler-Natta pipe resins. This again demonstrates that the resins of the invention exhibit higher viscosity at low shear rates and lower viscosity at high shear rates as compared to known PE100 resins. This means that the resins of the invention exhibit in combination not only improved processability, particularly for extrusion of pipes and fittings, but also improved resistance to sagging following pipe extrusion.

TABLE I LMW FRACTION HMW FRACTION RESIN BLEND p1 MI2 Density p2 HLMI Density MI5 HLMI Density 1-Hexene ESCR Bell wt % g/10′ Kg/m³ Mw/Mn wt % g/10′ Kg/m³ Mw/Mn dg/min dg/min kg/m3 g/Kg Mw/Mn h Ex. 1 0.55 185.0 972.5 2.6 0.45 0.24 926.0 3.7 0.63 22.0 952.5 5.0 10.5 >850 Ex. 2 0.54 200.0 972.7 2.5 0.46 <0.1 924.0 4.6 0.28 13.0 951.4 7.0 11.7 >500 Comp. Ex 1 0.58 103.0 970.8 0.42 0.19 931.5 4.7 0.84 28.0 956.9 3.0 11 <18 Comp. Ex 2 0.65 11.5 964.7 2.8 0.35 0.17 933.6 4.6 0.83 17.0 955.0 7.2 <18 Comp. Ex 3 0.75 3.3 961.1 2.9 0.25 0.30 934.8 4.7 1.50 22.0 955.1 4.5 <18 Comp. Ex 4 0.85 2.1 959.9 2.9 0.15 0.10 927.8 5.6 1.70 23.0 955.2 3.0 4.4 <18 Comp. Ex 5 0.80 2.1 959.9 2.9 0.20 0.16 927.8 4.4 1.00 17.0 949.1 9.0 4.6 <18 Comp. Ex 6 0.64 21.0 966.4 2.7 0.36 0.10 927.8 5.6 0.78 19.4 953.5 3.0 7.6 <18 Comp. Ex 7 0.50 185.0 972.5 2.6 0.50 0.39 935.2 4.6 0.52 15.9 956.2 3.0 10.9 <23 Comp. Ex 8 0.52 200.0 972.7 2.5 0.48 0.17 933.6 4.6 0.36 13.6 956.8 4.0 12.6 <23 Comp. Ex 9 0.56 45.0 968.5 2.7 0.44 0.17 933.6 4.6 0.48 13.3 954.3 9.2 <17 Comp. Ex 10 0.59 32.0 967.5 3.1 0.41 0.10 927.1 4.3 0.79 21.0 954.5 2.0 9.3 20 Comp. Ex 11 0.66 11.5 964.7 2.8 0.34 <0.1 923.5 0.82 17.5 950.3 7.0 6.4 50 Comp. Ex 12 0.49 200.0 972.7 2.5 0.51 0.17 933.6 4.6 0.23 8.3 955.2 11.7 <15 Comp. Ex 13 0.52 13.0 965.0 3.1 0.48 0.17 933.6 4.6 0.24 6.1 952.4 8.3 15–24 Comp. Ex 14 0.70 0.6 956.3 3.4 0.30 0.39 935.2 4.6 0.39 6.5 950.7 4 <17 Comp. Ex 15 0.84 0.4 954.8 3.3 0.16 0.22 920.0 4.0 0.62 8.5 950.2 4.0 3.8 <15

TABLE II μ2 η0.01 η1 dPa · s Pa · s Pa · s η0.01/η1 Ex. 1 17300 260000 25136 10.3 Ex. 2 19000 404630 35829 11.3 Comp. Ex. 1 16200 193190 20980 9.2 Comp. Ex. 2 21900 121160 19687 6.2 Comp. Ex. 3 20800 70359 13901 5.1 Comp. Ex. 4 20600 51105 12096 4.2 Comp. Ex. 5 23300 108610 17774 6.1 Comp. Ex. 6 19900 136230 19823 6.9 Comp. Ex. 7 19400 286100 28271 10.1 Comp. Ex. 8 19500 346120 33574 10.3 Comp. Ex. 9 21100 225350 28229 8.0 Comp. Ex. 10 18700 170350 22582 7.5 Comp. Ex. 11 21000 137710 20286 6.8 Comp. Ex. 12 22100 491710 42100 11.7 Comp. Ex. 13 28900 412620 39118 10.5 Comp. Ex. 14 35400 265750 28421 9.4 Comp. Ex. 15 32900 204300 25981 7.9

TABLE III LMW Fraction HMW Fraction (1st Step) (2nd Step) RESIN BLEND ESCR p1 MI2 Density p2 HLMI* Density* MI5 HLMI Density Hexene Bell Test FNCT wt % g/10′ Kg/m³ wt % g/10′ Kg/m³ dg/min dg/min kg/m³ g/Kg Mw/Mn h h Ex. 3 0.58 330.0 973.0 0.42 0.04 922.4 0.41 18.5 953.3 4 14.3 >500 627 Ex. 4 0.60 335.0 973.3 0.4 0.02 917.7 0.38 19.9 952.5 6 14.9 >500 >4820 Ex. 5 0.54 134.0 971.0 0.46 0.02 921.3 0.19 7.1 949.8 7 11.1 >400 1445 Ex. 6 0.60 251.0 973.0 0.4 0.05 916.4 0.62 24.2 951.8 9 13.0 >500 2479 Comp. Ex. 16 0.90 0.99 957.4 0.10 0.02 913.8 1.17 15.9 953.4 2 4.8 <16 Comp. Ex. 17 0.80 1.2 956.9 0.20 0.27 927.2 0.90 15.2 951.7 4.5 4.7 20–24 Comp. Ex. 18 0.60 66.6 968.9 0.40 0.02 939.2 0.28 11.4 958.5 10.4 <16 Comp. Ex. 19 0.53 160.0 972.1 0.47 0.02 934.1 0.14 5.7 956.0 12.5 16.25 6 *calculated

TABLE IV μ2 η0.01 η1 dPa · s Pa · s Pa · s η0.01/η1 Ex. 3 16900 455090 33241 13.7 Ex. 4 16200 525030 35742 14.7 Ex. 5 23100 734060 44582 16.5 Ex. 6 15800 348480 27667 12.6 Comp. Ex. 16 23700 103370 16586 6.2 Comp. Ex. 17 23000 179440 22649 7.9 Comp. Ex. 18 20100 553330 38758 14.3 Comp. Ex. 19 23800 825220 51787 15.9

TABLE V Example 7 Example 8 REACTOR 1 C₂ (% mol) 20.9 16.2 comonomer — — H₂/C₂ (% mol/mol) 0.0588 0.0427 T (° C.) 80 80 residence time (h) 1.82 1.83 REACTOR 2 C₂ (% mol) 12.80 14.97 C₆/C₂ (% mol/mol) 3.1 9.1 H₂/C₂ (% mol/mol) — — T (° C.) 75 75 residence time (h) 0.89 0.87

TABLE VI Comp. Example 20 Example Example (Commercial PE 7 8 100) LMW fraction (reactor 1) weight (%) 56.9 55.5 — MI₂ (g/10 min) 459 149 — Density (kg/m³) 974.2 971.0 — HMW fraction (reactor 2) weight (%) 43.1 44.5 — HLMI (g/10 min)* 0.03 0.04 — Density (kg/m³)* 919.4 919.6 — Polyethylene resin blend hexene g/kg 9 10 — MI₅ (g/10 min) 0.26 0.2  0.34 HLMI (g/10 min) 15.6 10.1 10.7 Density (kg/m³) 952.2 949.5 ~950**  μ₂ (dPa · s) 16,800 19,900 >21,000    η_(0.01) (Pa · s) 587,960 675,480 ≦200,000     η₁ (Pa · s) 39,722 43,426 — η_(0.01)/η₁ 14.8 15.6 <8  *calculated **commercial PE 100 resins usually have a density around 960 kg/m³, but contain carbon black; the density of the resin can therefore be estimated as around 950 kg/m³.

TABLE VII Comparative Example 20 (Commercial PE Example 7 Example 8 100) ESCR FNCT >4700 >4700 300 5 MPa, 80° C. (h) Notched pipe test 3081 >2182 1000 4.6 MPa, 80° C. (h) Creep Resistance 20° C. (h) at 13.0 MPa >2000 400 200 Rapid Crack −12.5 to −15° C. <−20° C. −5 to −15° C. Propagation Critical temperature (critical pressure 10 bars) Charpy Impact Energy 14 19 10 (kJ/m²) (−25° C.) 

1. A process for the preparation of a polyethylene resin having a bimodal molecular weight distribution which comprises: (a) contacting ethylene monomer and a first co-reactant selected from the group consisting of hydrogen and a comonomer comprising a C₃–C₁₂ alpha olefin in a first reaction zone under first polymerization conditions to produce a first polyethylene in the presence of a first catalyst system which includes a metallocene catalyst component comprising a bridged bis(tetrahydroindenyl) compound of the general formula R″(IndH₄)₂MQ₂ in which each IndH₄ is the same or different and is tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridge between the IndH₄ groups which comprises a C₁–C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is a hydrocarbyl having 1 to 20 carbon atoms or a halogen; and a cocatalyst which activates the catalyst component; (b) contacting ethylene monomer and a second co-reactant selected from the group consisting of hydrogen and a comonomer comprising a C₃–C₁₂ alpha olefin in a second reaction zone under second polymerization conditions to produce a second polyethylene different from said first polyethylene in the presence of a second catalyst system which includes a metallocene catalyst component comprising a bridged bis(tetrahydroindenyl) compound of the general formula R″(IndH₄)₂MQ₂ in which each IndH₄ is the same or different and is tetrahydroindenyl or substituted tetrahydroindenyl, R″ is a bridge between the IndH₄ groups which comprises a C₁–C₄ alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and a cocatalyst which activates the catalyst component, and (c) blending together said first and second polyethylenes to form a polyethylene resin comprising a blend of from 35 to 49 wt. % of a polyethylene fraction of high molecular weight and from 51 to 65 wt. % of a low molecular weight polyethylene fraction having a lower molecular weight than said high molecular weight polyethylene fraction, said high molecular weight polyethylene fraction comprising a linear low density polyethylene having a density of no more than 0.930 g/cm³, and a high load melt index, HLMI, of less than 0.6 g/10 min. and the lower molecular polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm³ and a melt index, MI₂, of greater than 10 g/10 min., said polyethylene resin having a density greater than 0.946 g/cm³, a high load melt index, HMLI, within the range of 1–100 g/10 min., a dynamic viscosity η_(0.01) measured at 0.01 radian/second, which is greater than 200,000 Pa·s, and a ratio η_(0.01)/η₁ which is greater than 8; wherein (i) η_(0.01) is the dynamic viscosity measured at 0.01 radian/second; and (ii) η₁ is the dynamic viscosity of the polyethylene resin measured at 1 radian/second.
 2. The process of claim 1 wherein said first polyethylene comprises a high density ethylene homopolymer, and said second polyethylene comprises a copolymer of ethylene and a C₃–C₁₂ alpha olefin comonomer.
 3. The process of claim 2 wherein said C₃–C₁₂ alpha olefin in said second polymer comonomer is selected from the group consisting of butene, hexane, hexene, methylpentene, octene and mixtures thereof.
 4. The process of claim 2 wherein said comonomer in said second polymer comprises hexene.
 5. The process of claim 2 wherein said first and second reaction zones are connected in series and said first polyethylene is supplied from said first reaction zone to said second reaction zone.
 6. The process of claim 5 wherein said first co-reactant is hydrogen and said second co-reactant comprises a C₃–C₁₂ alpha olefin.
 7. The process of claim 5 wherein said first co-reactant comprises a C₃–C₁₂ alpha olefin as a comonomer, wherein said first ethylene polymer is a copolymer of ethylene and said comonomer which is supplied from said first reaction zone to said second reaction zone and the second co-reactant in said second reaction zone is hydrogen, wherein homopolymerization takes place in said second reaction zone.
 8. The process of claim 1 wherein said first and second polyethylenes are blended together to form said polyethylene resin comprising a blend of no more than 45 wt. % of said high molecular weight polyethylene fraction and at least 55 wt. % of said low molecular weight fraction.
 9. The process of claim 1 wherein said first and second polyethylenes are blended together to form a polyethylene resin having a ratio η_(0.01)-η₁ which is greater than
 10. 10. The process of claim 1 wherein said first and second polyethylenes are blended together to form said polyethylene resin comprising a blend of no more than 44 wt. % of said high molecular weight polyethylene fraction and at least 56 wt. % of said low molecular weight fraction.
 11. The process of claim 1 wherein said polyethylene resin has a density of at least 0.954 g/cm³.
 12. The process of claim 1 wherein R″ in each of the metallocene catalyst components of subparagraphs (a) and (b) is an ethylene bridge. 